The invention relates to an apparatus for tensioning rectangular pieces of fabric, particularly for making screen printing stencils. The apparatus comprises a tensioning frame consisting of stretcher bars disposed in parallel pairs, which are displaceable relative to one another, and have longitudinally movable clamping points which clamp a piece of fabric along its edges.
Screen printing stencils, such as are used in screen printing on ceramic, glass and plastics substrates and also on a large number of other printable materials, consist of screen material, stencil material and frame.
In the production of screen printing stencils, the screen material, which usually consists of a fine-mesh plastic or wire fabric, is mechanically tensioned in the weft and warp directions and, while in the tensioned state, is joined to the stencil frame, for example by adhesive bonding. For this purpose, tensioning devices are used, which make it possible to place the screen material under the required tension and to hold it under tension until completion of the bonding process. The screen material is subjected to an excess of tension in relation to the area of material tensioned, and, after the setting or drying of the adhesive, the frame is separated from the area of screen material, whereupon the pattern or design is applied.
Important properties determining the quality of a screen printing stencil are the degree and uniformity of the tensioning of the screen material. In the screen printing operation it is well known that the stencil is not laid against the substrate to be printed, but is held at a certain distance (stand-off) from the substrate. The contact pressure of the ink transfer member, the squeegee, is now so selected that along its line of application the squeegee presses the screen material against the substrate to be printed such that at all other points the screen material is not in contact with the substrate. This arrangement ensures that the image finally printed will correspond as closely as possible to that formed on the stencil.
During printing with stand-off, the screen material undergoes a geometrical modification. Through the contact pressure of the squeegee, the screen is stretched out of the flat two-dimensional position into a three-dimensional state. During the movement of the squeegee an additional tensioning of the screen material, caused by the stand-off, is temporarily added to the tensioning of the screen material produced during the production of the stencil. If the two tensions together exceed the elastic limit specific to the screen material, they may lead to irreversible stretching of the screen material, resulting in a considerable fall in tension. The screen printing stencil becomes unusable in this case. While printing with stand-off is still possible with an irreversibly stretched screen printing stencil of this kind, the contact between the stencil and the substrate is no longer defined by the line of application of the squeegee. A smeared, unclear image is printed. This defect also occurs if the screen material did not have the required tension at the outset. Finally, unclear printirg faults occur if the screen material has uneven tensions.
To sum up, it may be said that uniform tensioning of the screen material, has a decisive influence on printing results. The construction of the tensioning device which can uniformly control the degree of tensioning for different materials is therefore of great importance.
Tensioning devices of the type customary at the present time comprise a tensioning frame consisting of stretcher bars disposed in parallel pairs which are displaceable relative to one another. Numerous grippers or clips are mounted for longitudinal movement on the stretcher bars, and hold the piece of fabric fast along its edges. For the tensioning of the piece of fabric in the warp and weft directions, the stretcher bars are moved crosswise mechanically or pneumatically.
The reason for the arrangement on each stretcher bar of a large number of individual grippers mechanically independent of one another is as follows: if the piece of fabric is tensioned, for example, in the warp direction, those grippers which hold the piece of fabric in the weft direction must follow the elongation of the piece of fabric which occurs in the warp direction in order to ensure that the fabric is tensioned with the threads straight, particularly at the edges of the material. The same applies to elongation in the weft direction, where the grippers working in the warp direction are concerned. This requirement is met by guiding the grippers on each stretcher bar on rails, often with the aid of ball bearings.
Depending on the size of the piece of fabric to be tensioned, a large number of grippers may be required, so that the cost of manufacture of the tensioning device is increased. In addition, each gripper must be correctly applied to the fabric and closed before the tensioning operation making the clamping a time-consuming operation.
Despite the numerous grippers, the edge of the fabric is held only at individual points, and the tensioning force is applied only at these points. Between the grippers there may be a considerable loss of tension in the fabric. Uniform tensioning of the material is therefore impossible in this manner.